1. Field of the Invention
The present invention relates to low temperature or cold food storage equipment and, more particularly, to low temperature food storage equipment with which it is possible to keep food in cold storage for a long period of time through utilization of low temperatures available during the winter in cold districts and to transport the food while keeping it cold in the storeroom without the necessity of transshipping it to a refrigerator truck or the like.
2. Description of the Prior Art
One method that has heretofore been used to keep food in cold storage is to utilize low temperatures available during the winter in cold districts. A simple storage facility that uses this method is a storage pit which is dug in the ground and utilizes latent and sensible heat of the surrounding frozen soil to keep food in cold storage for a long period of time.
There has been proposed the use of heat pipes for quickly forming a frozen soil layer around the storage pit as disclosed in Japanese Patent Laid-Open Publication No. 233957/90, for example. The cold storage equipment described in this Japanese document is made up of a water-barrier heat insulating layer provided in the ground, a food storeroom provided inside the heat insulating layer, and heat pipes having their lower end portions inserted into the ground between the water-barrier heat insulating layer and the stockroom. However, the cold storage equipment of this kind has technical problems described below.
With the cold storage equipment set forth in the above-identified Japanese document, food is kept at low temperatures in the underground storeroom during the winter, and hence in the case of shipment it must be carried out of the storeroom and loaded on a refrigerated truck or the like; this is very time-consuming and laborious work.
Moreover, the frozen soil layer, which serves as a cooling or chilling source, is frozen by letting in the cold or chilly outside air through the heat pipes, and hence its frozen state varies with the outside air temperature.